The present disclosure relates generally to Organic Rankine Cycle (ORC) systems and, more particularly, to a secondary cooling system therefor.
The Organic Rankine Cycle (ORC) is a vapor power cycle with an organic fluid refrigerant instead of water/steam as the working fluid. The working fluid is heated in an “evaporator/boiler” by a source of waste or low quality heat. The fluid starts as a liquid and ends up as a vapor. The high-pressure refrigerant vapor expands in the turbine to produce power. The low-pressure vapor exhausted from the turbine is condensed then sent back to the pump to restart the cycle.
The simple Rankine cycle used for power generation follows the process order: 1) Adiabatic pressure rise through a pump; 2) Isobaric heat addition in a preheater, evaporator and superheater; 3) Adiabatic expansion in a turbine; and 4) Isobaric heat rejection in a condenser, although other cycle modifications are possible such as the addition of a vapor-to-liquid recuperator.
ORC generators may be hermetic or non-hermetic. Non-hermetic generators do not share the working fluid environment and are typically cooled by air or water. Hermetic generators share and are cooled by refrigerant from the main cycle.
FIG. 1 schematically illustrates a conventional Organic Rankine Cycle system with standard hermetic generator cooling. A small secondary flow of liquid refrigerant is extracted from the pump exit and communicated to the generator casing. In this case the fluid is first fed through a stator cooling jacket and partially boils while picking up heat from stator losses. The remaining two-phase refrigerant is then sprayed into the rotor volume picking up heat from rotor and windage losses. The vaporized refrigerant is then vented downstream of the turbine exhaust.